Seems that the custom checker for testing import functions (custom, because they can’t be ordinarily run in the browser) relies on the import path being specified in double quotes. It should make no difference whether single or double quotes are used.
My initial answer:
import * as whatever_the_function from 'capitalize_strings';
…would not validate until double quotes were used on the path:
import * as whatever_the_function from "capitalize_strings";